This fixes some existing problems with the Variable::LinkedTo splitting/linking mechanism.  The problem was that if B is linked to A, and B needs a stack slot, but A doesn't get a stack slot, B's stack offset would never get initialized.  This could happen if A ends up with no explicit references in the code, or A's live range gets truncated such that it actually has a register while B doesn't.

It gets even more complicated if you have a link chain like A<--B<--C<--D etc. where some of them have stack slots (which should ultimately all be the same slot) and some don't.

The solution here is that if B is linked to the root A, and B has a stack slot but A doesn't, we can do a tree rotation so that B is the new root and A links to B.

In addition, we initialize Variable::StackOffset to an invalid value and always make sure a value used is valid.  Earlier attempts at extending the variable splitting would sometimes silently fail because the default StackOffset value of 0 ended up being used.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2116213002 .